Power generation in a nuclear reactor is accomplished by initiating a self-sustaining chain reaction. The amount of fissionable fuel used in the chain reaction is such that the multiplication factor (ratio of neutrons produced by fission in each generation to the number of neutrons in the preceding generation) can be made more than unity. To control this multiplication factor and accordingly the power output of the reactor, control or absorber elements are used to absorb neutrons within the reactor.
In the fast spectrum reactor context in which fast neutrons, as opposed to thermal neutrons, are used to sustain the chain reaction, the control assemblies are interspersed within a closely packed array of fuel assemblies. Both the fuel assemblies and the control assemblies are generally of a closed housing type in which the housings surrounding the fuel or control elements are provided with flow openings to permit coolant to flow therethrough. The coolant, which may be a liquid metal such as sodium, removes the thermal energy produced by the nuclear fissioning of the fuel.
The placement of the control assemblies within the array of fuel assemblies is such as to provide the most effective and efficient control of the reactor. Generally, this aided by providing three types of control assemblies. One type provides a general reactivity level control to regulate power output of the reactor. A second type provides fine control of reactivity within very small increments to compensate for drifts in reactor operating conditions. The third type of control elements rapidly reduce the reactivity level within the reactor to below the critical self-sustaining level in the event of certain particular malfunctions. These latter control assemblies are known as safety control assemblies since they act to rapidly shutdown the reactor. Also, the control elements of the other assemblies may generally be fully inserted simultaneously with the safety control elements if the reactor is scrammed.
Ordinarily the safety control element assemblies are comprised of a plurality of longitudinally extending parallel absorber elements adapted for longitudinal movement within the housing or duct. During normal reactor operation the elements of a safety control assembly are suspended as a unit inside the duct but above the fissile fuel zone. When the reactor is tripped or scrammed the safety elements are released by a latch at their upper end and are driven downward under the action of gravity. In some instances, a spring may be included to insure release from the latch and to initially accelerate the elements downwardly. When the latch releases, the safety element insertion is entirely controlled by conditions inside the duct, e.g., spring force, fluid drag, element weight, buoyancy and sliding friction. Accordingly, if severe duct distortions occur, the safety elements may become jammed in the duct and therefore not inserted.
Control assembly duct distortion can occur as a result of a variety of phenomena. Some distortion is inherent during reactor operation and generally may be predicted. Examples of such predictable duct distortion include bowing of the duct as a result of a temperature differential across the duct, nonsymmetrical neutron induced swelling and distortion of the duct material and differences in creep under stress. Since such distortions are predictable, the control assembly can be designed to accommodate this. This is what has been done in the past. However, duct distortion can also occur as a result of unforeseen and unpredictable phenomena, such as "denting" of the duct during fuel handling, adjacent fuel assembly failure, and failure of the radial core restraints which normally clamp the fuel and control assemblies together. As can be appreciated, in these instances it is still desirable, if not more desirable, to prevent the control or absorber elements from becoming jammed in the duct.
Prior art methods of accommodating possible unpredictable distortion have included providing more clearance between the absorber elements and the interior wall of the duct and/or placing a sleeve around the elements within the duct. However these method are not fully satisfactory since they result in an uneven distribution of the coolant around the element and reduction in neutron absorbing characteristics.